This invention relates to an electrical machine of the synchronous type whose stator winding is fed by a static converter, and it relates, more particularly, to a synchronous machine which is excited by permanent magnets within the rotor. There are pole gaps and the rotor has a soft-magnetic component on whose circumferential surface the permanent magnets are mounted directly adjoining the mechanical air gap.
An electrical machine of this type is described in the German Publication "etz", volume 100, issue 24, 1979, pages 1382-1386 (FIG. 3a, in particular). The electrical machine is the synchronous motor of a servo drive. Permanent magnets exciting the motor are mounted on the circumferential surface of the soft iron motor. The permanent magnets have no pole shoes and are thus directly positioned at the air gap. They are magnetized in a radial direction and their thickness, like that of the mechanical air gap, is constant. Similarly, the distance between the rotor core and the stator surface facing the rotor is also constant. The permanent magnets are made of an SmCo (samarium cobalt) material. The SmCo material is characterized by high remanent induction and high coercive field strength. When subjected to an external field, this material behaves similar to air in terms of its permeability.
The stator three-phase winding consisting of coils with an equal number of turns is fed by a static converter with sinusoidal-shaped three-phase currents. The stator currents produce a stator electric loading (average amphere conductors per cm of the air-gap periphery).
The currents are controlled depending upon the rotor position so that the electric loading is only routed along the q axis. Thus, a definitive correlation of the stator electric loading with the exciter field generated by the permanent magnets is established. This fixed correlation is comparable to the orientation brought about by the commutator and the brushes of the rotor electric loading to the stator-wide exciter field in a d.c. commutator machine with stator-wide excitation. As the material of the permanent magnets behaves similar to air towards the magnetic field (armature field) generated by the stator electric loading, an magnetic air gap is effective in developing the armature field, which magnetic air gap comprises the mechanical air gap and the magnet thickness.
The armature field formed influences the exciter field, which is generated by the permanent magnets, so that it is enhanced at one edge of the permanent magnet and diminished at the other edge. Aside from the danger of demagnetizing, the excessive field increase generated by the armature reaction requires higher voltage ratings in the converter components, such as transistors, thyristors, commutation devices, thereby necessitating added expense.
German Patent Document Offenlegungsschrift No. 2.637.705 also describes a small motor with permanent-magnet excitation in which the permanent magnets are arranged in the stator. The rotor has a commutator winding which is fed via brushes and commutator by a d.c. voltage supply. Thus, the rotor electric loading has a fixed relationship to the exciter field generated by the permanent magnets in the stator. The rotor electric loading generates a magnetic field (armature field) which, given high motor loads, can irreversibly demagnetize the permanent magnets at the trailing edges. To suppress the damaging effect of the armature field, the thickness of each permanent magnet in the direction of magnetization is chosen to increase from the entering edge to the trailing edge of the permanent magnet. This measure produces an approximately constant field pattern for a given load parameter in the air gap above the permanent magnets.
It is an object of the present invention to provide an electrical machine of synchronous design which reduces the voltage rating due to the armature reaction in the converter components.
Another object of the present invention is to reduce the overrating of the switching elements of the static converter due to the armature reaction in a static converter-fed electrical machine.